This invention relates generally to switches used in programmable controllers and, more particularly, to controllable switches that can be tested periodically to ensure more reliable operation. In many control systems, controllable switches are used to actuate devices such as motors, pumps, valves and so forth. Typically a switch is connected between a power source, which may be an alternating current (ac) source or a direct current (dc) source, and a load, such as a motor or a valve solenoid. In many applications of controllable switches, reliability is of prime importance and techniques have been devised to allow automatic testing of switches of this type. When a switch is in its "on" condition, testing usually involves momentarily turning the switch off and observing the expected cessation of current flow through the switch. When the switch is in its "off" condition, testing may involve momentarily turning the switch on and observing the expected current flow. Switches of the prior art have at least two drawbacks in relation to their testability.
Testing a controllable switch of the prior art requires that the switch be connected to its power supply or its load, or both. However, in many situations there may be additional "permissive" switches connected in series with the controlled switch. The state of these additional switches is, in general, unknown to the designer of the controllable switch and its testing procedures. Therefore, it is not always possible to ensure that the controlled switch is connected to its power supply or to its load. A second difficulty is that some switches of this general type are used to connect dc power sources to their loads and others are used to connect ac power sources to their loads. There are well known problems inherent to on-line testing of ac switches. In particular, the test results are sensitive to the ac phase angle at which the momentary test is performed. Ideally, what is needed is a switch that is testable regardless of whether or not the switch is connected to the load or its power source, and regardless of whether the power source provides alternating current or direct current. The present invention is directed to this end.